The present invention relates generally to materials and processes for sealing thin film batteries, and specifically to a hermetic seal comprising a barrier layer formed from one or more of tin oxide, tin phosphate, tin fluorophosphate, chalcogenide glass, tellurite glass, and borate glass. The materials and processes described herein can be implemented to significantly enhance the efficiency of thin film battery packaging, which can extend battery lifetimes and increase achievable energies and power densities.
Thin film batteries function much in the same way as traditional batteries, but can have a total thickness less than 1 mm (e.g., 0.35 to 0.65 mm) and are suitable, for example, for low-voltage (1.5 to 3.0 V) applications where traditional button cell geometries may be inadequate. Because thin film batteries are rechargeable, their size need be no larger than required to satisfy the energy requirements for a single discharge, thus reducing weight and cost. Further, compared to traditional lithium-ion batteries, which comprise a liquid electrolyte core, thin film batteries have a solid state core that obviates concerns over electrolyte leakage and makes thin film batteries less vulnerable to overheating. Thin film batteries have a large range of operating temperatures that can extend, for example, from as low as −20° C. to as high as 130° C., and can be repeatedly charged and discharged with minimal detrimental effect. Based on these and other properties, thin film batteries have tremendous potential for applications in microelectronics and MEMS industries as active or stand-by power sources.
Due to their ultrathin profile, low thermal mass, ability to operate across a wide temperature range, and fabrication compatibility with existing technologies, thin film batteries are well-suited for a variety of applications including, for example, power sources for diagnostic wafers for semiconductor processing, wireless embedded sensors, smart cards, active radio-frequency identification (RFID) tags, non-volatile memory backup, and implantable medical devices. These small-scale power systems have the potential to enable innumerable new wireless devices, and further applications include cell phones, laptop computers, personal electronic assistants and hybrid communication devices.
In semiconductor applications, thin film batteries can be deposited directly onto chips or chip packages in any desired shape or size. Power-paper batteries, for example, can be printed directly onto thin substrates such as paper, so they can be extremely flexible. Moreover, multiple thin film batteries can be fabricated into high density arrays of discrete units each having an areal footprint of, for example, about 0.25 to 1 mm2. Planar thin film batteries can be rolled or stacked into cylindrical or prismatic cell designs that can be packed together into modules. Such configurational flexibility allows multiple batteries to be connected in parallel or in series, depending on the application. By incorporating the barrier layer according to the invention, individually-packaged thin film batteries can be handled during and after device fabrication without adversely affecting failure rates.
Notwithstanding the foregoing, wide scale proliferation of thin film batteries depends on their meeting a number of additional challenges, including improved capacity, lower cycling losses, and increased lifetime. The lifetime of a thin film battery, for instance, can be affected by factors such as the thermal and electrochemical stability of the electrodes and the electrolyte, as well as the hermeticity of the battery package.
The desire for hermeticity is motivated primarily by the reactive nature of commonly-used alkali metal-based anodes, such as Li metal or Li6C and, in particular, the adverse reaction of these materials with air and water. Specifically, the reactions with air or water can create unsafe conditions and detrimentally compromise thin film battery performance. It is therefore desirable to minimize air or water exposure of the thin film battery layers and, in particular, the anode material both during and after fabrication.
Barrier layer materials and processes to provide barrier layer materials for hermetically packaging thin film batteries are known. A goal of such barrier layers is to significantly reduce oxygen and water permeability such that the anode remains unexposed and intact even at elevated temperatures and in humid conditions.
In view of the foregoing, it would be advantageous to provide an improved thin film battery barrier layer capable of protecting a lithium thin film battery under aggressive operating conditions. For example, conditions can include operation for 1000 hrs or longer in 85% relative humidity at 85° C.
These and other aspects and advantages of the invention can be achieved by a materials system and process for providing high performance encapsulation for thin film batteries. Such encapsulation can extend the lifetime of thin film batteries and particularly alkali metal-based thin film batteries by ensuring that the external environment is not the leading cause of failure. Specifically, high performance encapsulation layers according to the invention are incorporated into thin film batteries to hermetically seal the materials layers used to form the batteries and, specifically, alkali metal-based anode materials.
According to further embodiments, the choice of the barrier layer material(s) and the processing conditions for incorporating the barrier layer materials are sufficiently flexible that the thin film battery is not adversely affected by the barrier layer. Exemplary barrier layer materials include tin oxide, tin phosphate, tin fluorophosphate, chalcogenide glass, tellurite glass and borate glass. In embodiments, the barrier layers can be derived from room temperature sputtering of one or more of the foregoing materials or precursors for these materials, though other thin film deposition techniques can be used. Further, the deposition process can be tuned to control barrier layer thickness, conformality and stoichiometry. In order to accommodate various thin film battery architectures, deposition masks can be used to produce a suitably patterned barrier layer. Alternatively, conventional lithography and etching techniques can be used to form a patterned barrier layer from a uniform layer.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.